wgengine/magicsock: fix js/wasm crash regression loading non-existent portmapper

Thanks for the report, @Need-an-AwP!

Fixes #17681
Updates #9394

Change-Id: I2e0b722ef9b460bd7e79499192d1a315504ca84c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
(cherry picked from commit edb11e0e60)

ipn/localapi/debug.go

@@ -19,6 +19,7 @@ import (
 	"sync"
 	"time"
 
+	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/feature"
 	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/ipn"
@@ -39,6 +40,7 @@ func init() {
 	Register("debug-packet-filter-matches", (*Handler).serveDebugPacketFilterMatches)
 	Register("debug-packet-filter-rules", (*Handler).serveDebugPacketFilterRules)
 	Register("debug-peer-endpoint-changes", (*Handler).serveDebugPeerEndpointChanges)
+	Register("debug-optional-features", (*Handler).serveDebugOptionalFeatures)
 }
 
 func (h *Handler) serveDebugPeerEndpointChanges(w http.ResponseWriter, r *http.Request) {
@@ -463,3 +465,11 @@ func (h *Handler) serveDebugLog(w http.ResponseWriter, r *http.Request) {
 
 	w.WriteHeader(http.StatusNoContent)
 }
+
+func (h *Handler) serveDebugOptionalFeatures(w http.ResponseWriter, r *http.Request) {
+	of := &apitype.OptionalFeatures{
+		Features: feature.Registered(),
+	}
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(of)
+}